In order to establish a sustainable society, approaches to environmental issue or energy issue have been taken actively. Under these circumstances, expectations for thermoelectric conversion technology have increased. Since heat is the most common energy source that can be recovered from various situations such as body temperature, solar heat, engine, industrial exhaust heat and so forth, for the purpose of achieving high-efficiency energy utilization in a low carbon society, and feeding power to a ubiquitous terminal, a sensor and so forth, a thermoelectric conversion is expected to become increasingly important.
Generally, a bulk type thermoelectric device in which a thermoelectric couple module structure is built by processing and jointing a sintered body of a thermoelectric semiconductor material such as Bi2Te3 is commonly used. Recently, a thin film type thermoelectric device in which a thermoelectric semiconductor thin film is formed on a substrate using sputtering or the like has been developed, and the device is gathering attention.
Advantages of thin film thermoelectric conversion device are enumerated as follows, for example:
(A) small and lightweight,
(B) high productivity owing to batch deposition of a film with a large area by sputtering, printing and so forth,
(C) being able to reduce cost by using an inexpensive substrate,
(D) being able to implement a flexible thermoelectric device by adopting a substrate with high flexibility, and so forth.
Some methods using coating or printing are known as a method for manufacturing a thin film thermoelectric conversion device.
For instance, in Patent Literature 1, there is disclosed a method comprising coating a material obtained by mixing a powdered Bi2Te3 material with a binder so as to be made pasty on a substrate by a screen printing method or the like to form a thermoelectric device pattern.
In Patent Literature 2, there is disclosed a method for manufacturing a thermoelectric conversion module in which a thermoelectric device is formed by pattern-printing inks including a thermoelectric semiconductor material and an electrode material using inkjet method.
Further, In Patent Literature 3, there is disclosed a method in which, in a thermoelectric conversion device including a p-type semiconductor element, an n-type semiconductor element, and an insulator, at least one of a p-type semiconductor element and an n-type semiconductor element includes an organic semiconductor material, and the p-type semiconductor element and the n-type semiconductor element are formed by coating or printing.
It is also noted that there is a problem in which it is difficult for a thin film type thermoelectric device to generate and hold a temperature difference between a front surface and a rear surface of the thin film (thermoelectric semiconductor film) due to thickness of the thin film. In many cases for power generation utilization, a thermoelectric conversion using a temperature difference (temperature gradient) in a surface-normal direction of a thin film including a thermoelectric material is needed.
In a thin film thermoelectric conversion device, in the case where a thermoelectric semiconductor film is too thin, a thermal insulation is not sufficient and hence it is difficult to hold a temperature difference between a front surface and a back surface, thereby an effective power generation being disabled.
Therefore, a thickness of a thermoelectric semiconductor film preferably is of several tens of micron meter or more, for instance.
However, it is difficult to manufacture such a thick film thermoelectric couple structure by patterning using coating or printing process or the like. As a result, a tradeoff occurs between high efficient conversion and productivity.
In recent years, there has been found a spin Seebeck effect in which flows of electron spin are generated by setting a temperature gradient in a magnetic material. For instance, in Patent Literature 4 and non-Patent Literatures 1 and 2, there are disclosed thermoelectric conversion devices based on the spin Seebeck effect. These devices extract an angular momentum flow (spin flow) generated by the sin Seebeck effect as a current (electromotive force) by an inverse spin Hall effect (an effect in which when a spin flow is made to flow, a current flows in a direction perpendicular to the spin flow).
A thermoelectric conversion device disclosed in Patent Literature 4 is arranged by a ferromagnetic layer and a metal electrode deposited by a sputtering method. When a temperature gradient is given in parallel to a surface of the ferromagnetic layer, a spin flow is induced along a direction of the temperature gradient by the spin Seebeck effect, and with the induced spin flow, a potential difference is generated across both ends of the metal electrode by an inverse spin Hall effect in the metal electrode contacted with the magnetic layer, the potential difference being able to be extracted to outside as a current. This allows a temperature difference based power generation extracting an electric power from a heat to be realized.
In thermoelectric conversion devices disclosed in non-Patent Literatures 1 and 2, thermoelectric conversion devices are formed by a magnetic insulator and a metal electrode.
When the spin Seebeck effect is used, a complicated thermoelectric couple structure is necessitated. Thus, the above-mentioned problem regarding patterning of the thermoelectric couple structure is solved and it can be expected that thin film thermoelectric conversion facilitating a low cost and a large area is realized.
With a thermoelectric module using the spin Seebeck effect. an electric conductive portion (electrode) and a thermal conductive portion (magnetic material) are able to be designed independently.
Thus, a high efficient thermoelectric conversion device having the following structure is expected to be designed:
an electric conductivity being large (an ohmic loss being small); and
a thermal conductivity being small (being able to hold a temperature difference between a front surface and a back surface).